Sunday 6PM Update:  This afternoon’s NAM and GFS model runs continue with a significant snowstorm for us starting after 1 AM Tuesday and continuing through at least late morning on Tuesday.  Total QPF is about 0.80 (NAM) and about 1.00 inch (GFS) for Philadelphia.   With a 12:1 ratio, we’re talking about 9-12 inches of snow as a current estimate.  It’s expected to be very windy as well.  Updates about 10:30 tonight.

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Just a quick update:

The NAM has joined the GFS with a slightly more eastern track for the storm, putting us in the cold sector.   The storm looks like it will be a faster mover and not linger into Tuesday afternoon.

Current QPF values are about 0.75 inches water for the NAM and about 1.00 inches water for the GFS.  With a 12:1 ratio, we’re taking at least 8-10 inches for PHL, mostly occurring before daybreak and very early morning Tuesday.  Effects of the March sun through the clouds  won’t be a factor for much of the precipitation, but will allow for faster melting on roadways once the snow ends.    Temperatures remain below freezing.

The details really can’t be nailed down until Monday evening.  I’ll update later tonight. (The updates will be after 10:30 PM  and will only be for the NAM, with the GFS data coming out  an hour later.)

Friday 6 AM update:  Last night’s models bumped the QPF to about 0.35 inches water.  Snow accumulations more likely to be 2 inches around PHL, mostly on grassy surfaces, little remaining on roadways, mostly slush.

The latest NAM data has become available.  It’s QPF value has risen to 0.26 inches water for PHL.    Tonight’s GFS model data won’t be available until about 10:40 PM, so I’ll be basing this discussion on this afternoon’s GFS data.

There continues to be differences in the models-  The GFS starts the snow about 3 AM, while the NAM model doesn’t have the snow starting until about 7AM for the immediate PHL area.

The GFS has double the QPF of the NAM, or about 0.50 inches water.   That would make a big difference in the snowfall totals.

What’s interesting is that typically the NAM QPF is almost always higher than the GFS QPF.  But with this situation, it is reversed.    In past situations where the NAM QPF has been significantly lower than the GFS, the NAM has been correct.

Temperatures will be near or above freezing for most of the time. Ground temperatures are very warm.   Solar insolation through clouds in March has a significant negative effect on accumulations. As mentioned, predicting accumulations for daytime snows with low QPF values can be just an academic exercise in March.

So what does all this mean?  I’m going with the NAM model, although the differences between the models makes the confidence in this forecast lower than usual.

Here’s my best shot:  Snow starts just before 7 AM in PHL and the immediate suburbs, (earlier start further north from PHL)  and accumulates about 1-2 inches on grassy surfaces.  It will be a  wet snow.  Little lingering accumulation on roadways, especially as the morning progresses, although there will be periods of moderate snow between 7 and 10 AM which may be slushy at times.   Snow tapers late morning with lighter snow showers into early afternoon.

Areas north of PHL will have more accumulation, possibly 3-4 inches, mostly on grassy surfaces.

It will become windy and cold during the afternoon and wet areas will freeze during the evening.